Web Mapping

INTRODUCTION:
           The term "Web mapping" constitutes both the technology and art of sharing maps on the Internet.

The simplest Internet-based maps are static, such as images (i.e., jpegs or tiffs) that do not allow users to change the components, extent, or appearance of the map. These maps are the easiest and simplest to share because all that is required is placing the image on a Web server and telling others where to find it. At the opposite end of the spectrum are customizable, interactive Web-based maps. These maps are usually created with the use of modern hardware, complex software. In addition, these initiatives require human expertise for successful implementation.

The advantage of developing and implementing interactive maps is that they give the user the ability to customize the map to meet this own specific needs. Possible user interactions include adjusting the extent of the map by zooming in or out, turning on and off features displayed within the map, and locating specific areas such as by entering a ZIP code or a street address.

TYPES OF WEB MAPS:              

QUERYMAP:

Query maps allow users to see what their query results look like in map form.  To set a query map, you need to add the QUERYMAP object in your map file.  It’s been done for you but do have a look at it—it’s a simple but very useful object.

QUERYFILE:

The second map generated by your query comes from the QUERYFILE.  QUERYFILE is a Map Server CGI variable (not a map file object) and is generated By Map Server using the SAVEQUERY variable.  When SAVEQUERY is given a Value of “true”, Map Server generates a QUERYFILE which can then be used to create a query map on subsequent queries.

Example:

Projections:

Group/Apache2/htdocs/ms_workshop/example4.map&layer=states&layer=county&layer=fedland&layer=water&layer=mod_evi&mode=browseThis is what our application would have been like if we didn’t use on-the-fly re projection.  For the next 30 minutes or so we will learn about on-the-fly re projection capabilities of Map Server. Map Server works with projected and un projected (geographic) datasets.  However, for Map Server to calculate scales and create scale bars properly, the dataset has to be projected in a grid-based coordinate projection system (i.e. UTM, Lambert, and Albers).  Map Server can re project geographic data on-the-fly, with the help of the Proj.4 library.  In addition, the GDAL library provides support for raster file re projection.  Be aware that on-the-fly re projection will result in less than optimal performance for Map Server.  Re projection can be a compute-intensive process, and can slow Map Server’s output creation substantially. Having said that, on-the-fly re projection can still be very useful—you can re project your data to any projection you want.  You might want to create an application where the client chooses the projection and Map Server generates the proper map for that particular client.  If you want to create a WMS-compliant application, you must specify your spatial reference system (SRS).  You’ll need projection support for that SRS to be recognized by Map Server. We can set our map file PROJECTION object using either “raw” parameters or specifying an EPSG reference number.  This “reference system” was created by the European Petroleum Survey Group (EPSG, hence the name) and contains geodetic parameters for several projections.  It’s become a standard reference system, used in WMS applications.  Unfortunately, most of the projections defined in the EPSG file are for smaller areas (larger scale maps) than the upper great lakes.  You could use UTM projection for your application but the upper great lakes region cover two UTM zones.  What I have done is what ESRI and other companies have done—creates custom projection definitions.